Put ext_implicit_lib_function_decl in ImplicitFunctionDeclare.

If we build with -Werror=implicit-function-declaration, only implicit
function declarations of non-library functions throw compiler errors.
For library functions, we only produce a warning. There is no way to
promote both of these cases to an error without promoting other
warnings.

It makes little sense to introduce an additional compiler flag just to
control this specific warning. In my opinion it should just be part of
the same group.

llvm-svn: 246857